Small Wars Journal explains the military reason for freeing the slaves

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In July 1862, after a series of Union military setbacks, President Abraham Lincoln wrote that slavery was the “the heart of the rebellion.”[1] Lincoln recognized how slavery provided the South with cheap labor that could sustain Southern agriculture and industry while the white population went to war.[2] Slaves also provided valuable military service, constructing fortresses and infrastructure that enhanced Confederate combat power and mobility. Destroying slavery, Lincoln understood, could deny the South a valuable source of productive strength that would reduce the Confederacy’s military and economic power.

Over the next three years, Union military strategy evolved from its initial conciliatory phase to the “hard hand” phase that sought to destroy Southern military power—a power that owed much to the institution of slavery.[3] The most famous campaign of the “hard hand” phase of the war was General William T. Sherman’s 1864-1865 campaign in Georgia and the Carolinas—the birthplace of the insurrection. The campaign, which started with the burning of Atlanta and ended with a furious, vengeful march through the Carolinas, brought the Union Army into some of the South’s largest slave plantations. The presence of Sherman’s armies inspired countless blacks to flee slavery into Union lines. This influx of blacks into Sherman’s ranks enhanced Union intelligence and combat power, while dealing a devastating blow to the South’s ability and will to fight. Sherman’s campaign, therefore, helped plant a knife through the heart of the rebellion.

When Sherman entered Georgia in mid-1864, his three armies inspired and enabled countless slaves to escape from bondage into the Union lines. Thousands of these escaped slaves crowed into the rear of Sherman’s advancing armies.[4] So long as they remained within Union lines, where they could evade recapture, they were free—a freedom legally guaranteed by President Abraham Lincoln’s 1863 Emancipation Proclamation that declared all blacks living in states that remained in open rebellion “then thenceforward forever free.”[5] Entering Union lines offered more than protection and freedom – it also offered promises of economic gain. In January 1865, General Sherman, in the aftermath of his famous march to the sea from Atlanta, issued Special Field Order 15. This order set aside large swaths of fertile, coastal land that stretched from north Florida to South Carolina “for the settlement of the negroes now made free.”[6] Around 20,000 blacks settled this land by the end of the war.[7] In return for promises of freedom, economic gain, and an opportunity to undermine the society that had placed them in bondage, slaves provided vital support to the Union Army that helped quell the Southern insurrection.

Escaped slaves provided Sherman’s Army with critical intelligence on the physical and political landscape of its area of operations.[8] Moving armies over long distances in an era when maps and roads were highly unreliable was a significant challenge to military commanders. However, Sherman’s campaign benefited from the influx of escaped slaves, many of whom had lived and worked their entire lives in the fields and towns of Georgia and the Carolinas. These escaped slaves likely provided critical intelligence on the locations of river crossings, impassable terrain, water sources, etc. In addition to providing support with navigation, escaped slaves probably had some awareness of who were rebel sympathizers. These sympathizers, throughout the war, aided or participated in guerilla raids that harassed Union lines and drained valuable resources from the front. Information on who supported these raids would have helped Sherman’s armies implement more effective counterinsurgency operations. It would also have allowed the Union forces to focus the intensity of their “hard war” policies on the rebellion’s strongest sympathizers, while sparing loyal Unionists. Without the aid of escaped slaves, who served as important intelligence assets, the Union Army’s march across Georgia and into the Carolinas would likely have moved much slower and may have encountered more guerilla resistance.

While ex-slaves helped navigate the Union Armies across hostile, unpredictable terrain, other ex-slaves provided valuable combat support services that enhanced the mobility and combat power of Sherman’s Armies. Although Sherman himself was not a proponent of emancipation, he recognized the value blacks could provide his army in combat support.[9] Union Armies, by late 1864, occupied over 100,000 square miles of enemy territory—territory that drained critical manpower and resources to hold and tame.[10] Taming the land often required large garrisons of troops, reducing the amount of combat forces at the disposal of commanders in battle. Escaped slaves offered Sherman and other commanders a solution to this problem. Instead of using professional soldiers to garrison forts and protect their lengthy, vulnerable lines of communication, Union commanders utilized escaped slaves and, later in the war, black soldiers who were increasingly coming into their lines as the war progressed.[11] While on the march, Sherman’s armies used the ex-slaves to drive wagons, herd livestock, chop wood, and construct and repair transportation networks.[12] With an increased supply of labor, provided by escaped slaves, Sherman’s Armies were able to better supply their armies and concentrate more combat power in battle, rather than diffusing it to protect the ever-expanding Union lines of communication.

Each slave removed from bondage decreased the power of the Confederate Army. Slaves were critical resources for the South; they helped maintain its ability to wage war and sustain its economy. Slaves worked the fields, produced ammunition, and built fortifications, enabling white, male Southerners to fill the army’s ranks. But, by 1864, slaves were becoming scarcer resources; Georgia alone had lost over 60,000 slaves.[13] Without this pool of cheap labor, whites would have had to fill these important economic and military support roles, reducing the manpower allotted to the front line.[14] Southern farmers, by late 1864, worried they would be unable maintain their agricultural output, leading to political pressure on the Confederate government.[15]....